A conventional dishwasher and dryer shown in FIG. 3 accommodates tableware 48 placed in a dish basket 3 in a washing vessel 2. In operation, the operation is started by applying detergent, then closing the door, and pressing a start button.
When a water feed valve 15 is operated to open, water is fed from a tap connection portion 14 through a water feed pipe 16 into washing vessel 2 via a water feed inlet 17, and is accumulated above the level at which a heater 4 is soaked. When the feed water is accumulated to the level at which washing can be performed, and a not-shown level sensor senses the level, a control circuit 12 stops the operation of opening water feed valve 15 to stop feeding water and automatically carries out a series of subsequent operations.
Upon the water feed stop, a motor 8 starts forward rotation and an impeller of a pump 7 connected to a motor shaft also starts rotation. Therefore, the pump pressure-feeds water from the bottom portion of washing vessel 2 through a suction pipe 6 into a washing and discharging pipe 9. The water is further supplied from discharging pipe 9 to a rotary nozzle 11, is sprayed vigorously from a nozzle opening to tableware 48 placed thereabove, and is repeatedly circulated.
Here, heater 4 is powered as necessary to heat the feed water. Accordingly, the effects of the mechanical force of the washing water sprayed from rotary nozzle 11, the heat, and the detergent cause the dirt to be detached, dissolved, and decomposed from the dishes, thereby keeping the dishes washed. When the washing water attains a set temperature or when a set time has elapsed, the washing step is ended by rotating motor 8 backward to discharge the dirty washing water in washing vessel 2 from pump 7 through a drain discharge pipe 10 to the outside of the washing vessel via a drain outlet 10. Then, water feed valve 15 is operated to open to feed fresh tap water, followed by a rinsing step.
In the rinsing step, the dirt and detergent left on the dishes and the washing vessel is let out and rinsed by repeating several times a step of rotating pump 7 forward to spray water from nozzle for rinsing and thereafter rotating pump 7 again backward for discharging the water. At this time, in the final rinsing step, the water is heated by the heater to a high temperature for rinsing.
Once all the rinsing steps are ended by discharging the water in the final rinsing step, a not-shown air-blowing fan starts its operation, thereby starting a drying step. External air is blown into the washing vessel and exhausted through an air outlet provided in the vicinity of the door. In addition, heater 4 is operated on and off to dry tableware 48 and the inside of the washing vessel. Upon the end of the dry operation for any given time, the operation of the air-blowing fan and the heater terminates, whereby all the steps are ended.
In recent years, with the growing environmental awareness, environmental burdens of detergent drainage are taken seriously. When a dishwasher and dryer washes the dishes without detergent, starch dirt, among dirt such as starch, protein, and oils and fats, can be washed away only with the mechanical force and heat, taking much time at an appropriate temperature.
While the oils and fats can also be separated from a dish with an increased temperature, without detergent, they reattach to the dish, causing slimming after washing or fogging on a glass product. Moreover, unlike the other dirt, the protein dirt such as egg degrades to harden with a higher temperature, which is hardly washed away without detergent.
When tap water is used for rinsing, hardness components including Ca ions, Mg ions, and the like in the tap water form calcium carbonate or the like, which is whitely left on the surface of dishes, causing water spots. In Europe where the hardness of the tap water is particularly high, water spots are more likely to occur. Then, in Europe, a dishwasher and dryer that uses an ion-exchange resin to soften the tap water to be fed is commercially available.
The tap water in Japan is soft water containing less hardness components as compared with Europe. Yet, even a small amount of dirt such as oil and fat components left on a dish surface causes the binding with the hardness components, readily forming water spots. Accordingly, Japanese Patent Laying-Open No. 2000-300494 proposes a structure having an ion-exchange apparatus provided in a water feed path to soften the fed tap water in one or both of the washing step and the final rinsing step.
Furthermore, Japanese Patent Laying-Open No. 2001-238845 proposes a structure further including regenerating means in a water-softening apparatus for regeneration prior to the final rinsing-step. In any case, tap water is softened for washing and rinsing. In the latter case, the hardness component released from the water-softening apparatus using a regeneration agent at the time of regeneration is only discharged, and the hardness component and Na ions in salt are not utilized for washing.